Early season cover crop interseeding in corn

Cover crops can provide growers with many benefits, including erosion control, reduced offsite movement of nutrients, support for the development of soil organic matter and, in some cases, additional revenue streams (e.g., forages, oilseeds).

However, it’s estimated that less than two percent of the agricultural land in Minnesota uses cover crops at some point in the rotation. There are several reasons for low adoption rates, but the most obvious reason is a reduced growing season, which creates many challenges when trying to successfully integrate cover crops.

Potential of interseeding

New technologies can overcome these challenges, the most notable being early- or late-season cover crop interseeding (i.e., relaying cover crops into cash crops).

Interseeding cover crops during the corn or soybean growing season can address some of the potential challenges (e.g., soil moisture and light) associated with shorter growing seasons.

The University of Minnesota Cover Crop team and Soil Health Partnership are researching innovative techniques and equipment necessary to interseed cover crops in corn and soybean.

Similarities between weeds and cover crops

When considering interseeding classical cover crops (e.g., rye, clover, radishes, etc.), it’s a good idea to review or at least acknowledge some of the core weed science ideas due to the similarities between weeds and cover crops.

This isn’t to say that the vast majority of disciplines associated with crop production -- most notably breeding, genetics, entomology, plant pathology and soil fertility, to name a few -- aren’t central to advancing cover crops.

Figure 1: How the critical weed-free period impacts crop yield

Critical weed-free period

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It’s well known that weeds, when allowed to to compete with the cash crop, can reduce crop yields. An important concept in weed management is the idea of the critical weed-free period.

This critical period during the growing season is when weeds are most likely to impact crop yields (Figure 1). Weed control efforts outside the critical period offer no yield advantages, as long as early-season weeds are controlled before the critical period begins. These critical periods of weed control are unique for each crop.

Unfortunately, the critical period widely varies depending on the specific conditions in each field. Most agree the critical period of weed control in corn is between the V1 and V10 leaf stages.

Because cover crop interseeding involves additional field operations, economically speaking, it makes sense to combine cover crop interseeding with scheduled cash crop operations or tasks (e.g., planting cover crops at sidedressing). However, the critical weed-free period should be considered when introducing cover crops into the developing crop.

Like weeds, both crop and cover crop productivity directly relates to the ability to sequester nutrients, water and light. In this context, an early-planted cover crop could be considered as a weed.

Optimum time for interseeding cover crops

Most scientific journal articles suggests that early cover crop planting dates that align with the V5 to V7 corn leaf stage have negligible impacts on corn yield.

The Minnesota Department of Agriculture-funded cover crop interseeding project has demonstrated similar trends, where the no-cover-crop check plot yielded the same as the cover crop plots across the last two years. Plus, there have been no yield reductions associated with the following no-till soybean crop (Figure 2).

Targeting the V7 corn growth stage can position the cover crop planting date near the end of the critical weed-free period in corn (Figure 1). More research is needed to determine how early cover crops can be interseeded into corn without impacting yield.

In addition to interseeding dates, the following cover crops and seeding rates were used:

Cereal rye (aka winter rye): 120 pounds of live seed per acre.

Medium red clover: 10 pounds of live seed per acre.

Hairy vetch: 25 pounds of live seed per acre.

Annual ryegrass: 25 pounds of live seed per acre.

After each seeding event, there was sufficient rain, which greatly impacted the germination and establishment of the cover crops. In early July, the corn across the three cover crop planting dates showed no visible stress.

It’s important to note that the site has high mineralization potential. Coupled with regular rain events, this may mask any weedy interference associated with the V2 planting date.

As plots progress through the season, there’s a good chance that V2 treatments will reduce grain yields. The photos in Figures 4 and 5 were taken June 10, 2016.

Interseeding strategies

Much of what we understand about cover crop interseeding in the Upper Midwest is still in the research phase. More time and experiments are needed to fully predict how cover crops will impact cash crop performance across a wide range of environments.

Here’s some advice for those interested in planting cover crops:

Set low expectations and be conservative. With all systems, there’s associated risk and cover crops are no different.

Determine which services most interest you (e.g., erosion control, reducing offsite nutrient movement, grazing, etc.). This will help you decide which cover cropping system best fits your cropping system needs.

Have a plan. For example, if cover crops are seeded into soybean and they’re winter-hardy, what’s your plan for planting corn, not only into high residue, but also void of fall tillage? Strip tillage is a viable option for many corn and soybean growers.

Be prepared to move quickly in the spring. Cereal rye can quickly add biomass in the spring and it can be tricky to terminate legume cover crops that survive the winter with herbicides.

Start with the basics. Cereal rye is bulletproof and relatively inexpensive. It establishes well in the summer and fall, is winter-hardy and is easier to terminate in the spring than the legumes.

If you’re new to cover crops or any new technology, pick a uniform spot in the field and plant both with and without cover crops. Keep everything else constant. With yield monitors and adequate replication, you can assess if cover crops are a good fit for your farm.

Reach out for help and advice. There are several resources in Minnesota that can address your questions: University of Minnesota Extension, Natural Resources Conservation Service (NRCS), Minnesota Department of Agriculture (MDA) and U.S. Department of Agriculture (USDA), to name a few.